Ion implantation systems or ion implanters are widely used to dope semiconductors with impurities in integrated circuit manufacturing, as well as in the manufacture of flat panel displays, for example. In such systems, an ion source ionizes a desired dopant element, which is extracted from the source in the form of an ion beam of desired energy. The ion beam is then directed at the surface of the workpiece, such as a semiconductor wafer, in order to implant the workpiece with the dopant element. The ions of the beam penetrate the surface of the workpiece to form a region of desired conductivity, such as in the fabrication of transistor devices in the wafer. The implantation process is typically performed in a high vacuum process chamber which prevents dispersion of the ion beam by collisions with residual gas molecules and which minimizes the risk of contamination of the workpiece by airborne particles. A typical ion implanter includes an ion source for generating the ion beam, a beamline including a mass analysis magnet for mass resolving the ion beam, and a target chamber containing the semiconductor wafer or other substrate to be implanted by the ion beam, although flat panel display implanters typically do not include a mass analysis apparatus. For high energy implantation systems, an acceleration apparatus may be provided between the mass analysis magnet and the target chamber for accelerating the ions to high energies.
Conventional ion sources include a plasma confinement chamber having an inlet aperture for introducing a gas to be ionized into plasma and an exit aperture opening through which the plasma is extracted to form the ion beam. One example of a gas is phosphine. When phosphine is exposed to an energy source, such as energetic electrons or radio frequency (RF) energy, for example, the phosphine can disassociate to form positively charged phosphorous (P+) ions for doping the workpiece and hydrogen ions. Typically, phosphine is introduced into the plasma chamber and then exposed to the energy source producing both phosphorous ions, hydrogen ions and electrons. The plasma comprises ions desirable for implantation, phosphorous ions, into a workpiece, as well as undesirable ions, hydrogen ions, and electrons which are a by-product of the dissociation and ionization processes. The phosphorous ions and the hydrogen ions are then extracted through the exit opening into the ion beam using an extraction apparatus including energized extraction electrodes. To exclude unwanted ions like hydrogen, most ion implanters employ mass analysis apparatus, which is usually done with an aide of magnetic field created by electromagnet. Examples of other typical dopant elements of which the source gas is comprised include phosphorous (P), arsenic (As), or Boron (B).
The dosage and energy of the implanted ions are varied according to the implantation desired for a given application. Ion dosage controls the concentration of implanted ions for a given semiconductor material. Typically, high current implanters are used for high dose implants, while medium current implanters are used for lower dosage applications. Ion energy is used to control junction depth in semiconductor devices, where the energy levels of the ions in the beam determine the degree of depth of the implanted ions. The continuing trend toward smaller and smaller semiconductor devices requires a beamline construction which serves to deliver high beam currents at low energies. The high beam current provides the necessary dosage levels, while the low energy permits shallow depth ion implants. In addition, the continuing trend toward higher device complexity requires careful control over the uniformity of implantation beams being scanned across the workpiece.
In many ion implantation systems, a cylindrical ion beam is imparted onto a wafer target through mechanical and/or magnetic scanning, in order to provide the desired implantation thereof. Batch implanters provide for simultaneous implantation of several wafers, which are rotated through an implantation path in a controlled fashion. The ion beam is shaped according to the ion source extraction opening and subsequent shaping apparatus, such as the mass analyzer apparatus, resolving apertures, quadrupole magnets, and ion accelerators, by which a small cross-section ion beam (relative to the size of the implanted workpiece) is provided to the target wafer or wafers. The beam and/or the target are translated with respect to one another to effect a scanning of the workpiece. However, in order to reduce the complexity of such implantation systems, it is desirable to reduce the scanning mechanisms, and to provide for elongated ribbon-shaped ion beams. For a ribbon beam of sufficient longitudinal length, a single mechanical scan may be employed to implant an entire wafer, without requiring additional mechanical or magnetic raster-type scanning devices, for example.
FIG. 1 (See U.S. Pat. No. 5,350,926) illustrates a typical prior art broad ribbon beam ion implanter 100 for the implanting of silicon wafers which employs two separate magnets to produce a ribbon-shaped ion beam. Mass analysis of the ion beam, carried out with the first magnet and the second magnet, is utilized to distribute the ions in a more parallel pattern. The two magnetic system can create a uniform ion beam with adequate implantation purity and is a popular system within the ribbon-beam ion implantation industry. The prior art ribbon beam implantation system 100, however, suffers several difficulties when attempting to attain the ribbon beam current densities, energy and uniformity, for example. Typically, this system 100 necessitates a wide spatial footprint because of the two separate magnet assemblies, the optical cross over of the beam, the system is complex and the various components used to resolve the ion beam make the system expensive. Additionally, in this architecture, there is a sacrifice of beam transmission at low energy “drift” mode, since the beam line is much longer.
Another prior art broad ribbon beam system is illustrated in FIG. 2, (See U.S. Pat. No. 5,834,786) where the system 200 employs an optics architecture that utilizes a single magnet to form a parallel beam out of a small diverging ion source and also achieves mass analysis. This type of implantation apparatus and system has been typically used in implanting flat panel displays with uniform ribbon beams. Use of the ribbon-beam ion implanter 200; however has challenges with respect to inadequate mass resolving power.
Another conventional prior art broad ribbon beam technique is employed in an ion implantation system 300, wherein the ion source is as wide as the final width of the beam, and a single magnet assembly 302 is designed not to disturb the starting beam 304 parallelism out of the extraction system. The magnet assembly is designed so that each coil 306 and 308 is wrapped around a yoke 310 to create a uniform magnetic field across the wide direction of the gap, and the ion beam 304 is bent up or down according to the mass of the ions and by utilizing a horizontal aperture so that the mass analysis is accomplished. The magnet assembly 302 is designed solely for mass analysis alone and does not have any focusing effect in the direction perpendicular to the bending direction (side-side direction in FIG. 3). To obtain a parallel broad beam on the exit side of the magnet assembly, the beam 304 has to enter the magnet assembly 302 in a parallel fashion. Otherwise, it requires another focusing element after the magnet 302 to convert the non-parallel beam into a parallel beam. The above solution, however, requires that the beam enter the magnet as a wide parallel beam and that can restrict the kinds of ion sources used.
The magnet assembly used in FIG. 3 could be replaced with other types of magnet assemblies, providing the magnet has sufficient gap width to allow the entire beam to pass through it and the field is uniform across the gap. For example, a conventional dipole magnet 400, shown in FIG. 4a may be employed if the gap is expanded, however, a wide gap dipole magnet assembly tends to make the field unacceptably non-uniform, as shown. The magnet assembly shown in FIG. 3, is shown in cross-section in FIG. 4b, at 420. Two coils 422 and 424 wrap around two return yokes and the field within the gap is now very uniform. However, a serious drawback of this magnetic configuration is that it creates an enormous leakage field outside of the magnet assembly. The power necessary in this configuration has to support the unused magnetic leakage in addition to the useful magnetic field in the gap and therefore the efficiency is poor. Another type of magnet, typically referred to as a window frame magnet can provide a uniform magnetic field across a wide gap without creating unacceptable leakage field outside of the magnet assembly. A prior art window frame magnet assembly 400, is illustrated with reference to both FIG. 4 and FIG. 5. This basic window frame magnet could be configured with a wide gap as illustrated in FIG. 4c and be employed in a ribbon-beam ion implantation system as shown in FIG. 5. Referring to FIGS. 4 and 5, the window frame magnet assembly, 400 and 500 is constructed in which traditional pole pieces are missing and two coils, 402 and 404, occupy the area on either side of the active area 406, enclosed by a iron yoke 412. A positive current, current as seen into the paper in FIG. 4, is running through the coil 402, whereas a negative current, current as seen as out of the paper in FIG. 4, is driven through coil 404. For clarity purposes, no current return paths are shown in FIG. 4.
One draw back to the magnet assembly 500 is the window frame magnet assembly 500 bends the ion beam in only one direction and similarly to the case in FIG. 3, the starting beam that enters the magnet assembly 412 has to be parallel and wide to obtain a parallel wide beam on the exit side.
Accordingly, it is desirable to provide a single magnet assembly for creating a broad ribbon-shaped ion beam with improved mass resolution profile properties for use in such broad beam ion implantation systems.